This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Populations consuming high levels of soy, as in many Asian countries, have lower levels of ovarian hormones, lower rates of breast cancer, and reduced mammographic density than populations consuming typical Western diets. We found that soy feeding favorably lowered ovarian hormones by 25-45% in premenopausal women. We propose to test the hypothesis that consumption of soya reduces ovarian steroid hormones (estradiol, estrone, sulfate, and progesterone) and results in a corresponding reduction in breast density and that these effects can be achieved with a soy diet that does not contain isoflavones. Aim 1 determines the assess potential adverse effects of chronic soy exposure. The aims will be tested by conducting a randomized, double-blind study, with two arms and 100 premenopausal women in each arm. Premenopausal women with >50% of dense breast tissue will randomly allocated to take soymilk without isoflavones (treatment) or cow milk (placebo) as dietary supplements daily for 2 years. The comparative levels of ovarian steroids, hormones and breast density will be assessed before, during and after 2 years of dietary interventions in these two groups of women. Reducing breast density can be expected to reduce breast cancer risk by two mechanisms, i.e., reducing the volume of target tissue at risk for cancer development and improving the sensitivity of mammography for early screening detection. One of our long-term objectives is to determine if soy can be an alternative and complementary preventive measure for a second breast cancer.